Electronic devices are used in which electronic components such as a semiconductor element, a heat element, and a Peltier element are mounted on a circuit board with a metal wiring layer or the like bonded onto the circuit board therebetween. These electronic components generate heat when operating, and the amount of heat applied per unit volume of the circuit board is being increased as a response to recent demands for high integration of electronic components and size or thickness reduction of electronic devices. Accordingly, it is desired not only that the bonded members of the circuit board do not separate, but also that the circuit board exhibit high heat radiation.
Accordingly, through holes are formed in the support substrate of the circuit board, under the electronic components, and conductors (hereinafter referred to as through conductors) are formed in the through holes to increase heat radiation. However, if the through conductors are each formed by filling a through hole with a metal by plating, the metal grows from the inner wall of the through hole and finally forms a depression at the center of the surface of the through conductor. If the through hole is filled with a metal paste, a depression is formed at the center of the surface of the through conductor by shrinkage caused by firing. Also, the metal wiring layer covering the surface of the through conductor has a surface having similar characteristics to the surface of the through conductor. Accordingly, a depression is also formed in the metal wiring layer. This causes connection failure between the members when an electronic component is mounted on the metal wiring layer with an electrode pad therebetween, or heat radiation is reduced because the bonding area is reduced.
In order to solve such a disadvantage, the following PTL 1 proposes a non-shrinkable through-hole-filling conductive paste that will be fired after filling the through holes. The non-shrinkable conductive paste contains a conductive powder mainly containing a metal powder, an expanding agent, and a vehicle.